Composite materials, such as fiber-reinforced composites, can be used to produce corrosion resistant and lightweight structures. “Composite material,” as used herein, refers to a material comprised of two or more separate materials, which may include a fiber or polymer binder or a combination of the two. The most common composites are normally comprised of a fiber (Glass, Kevlar, Carbon, etc) impregnated with a polymer (Epoxy, Polyester, Urethane, etc). In comparison to lightweight metals such as aluminum, structures formed of composite materials have high strength-to-weight and high stiffness-to-weight ratios. As a result, composite materials have been used to fabricate a wide variety of structures including, most notably, aircraft structures.
In the aircraft industry, composite components were initially limited to secondary structures such as floorboards and engine cowlings due to limited experience with designing composite structures. However, as the mechanics of composite materials became better understood and higher quality materials were developed, its use increased as primary aircraft components such as flaps, wing sections, and even as the entire fuselage.
Currently, aircraft exist that have a fuselage and wings made substantially or entirely from composite materials. Aircraft manufacturers have increased their dependence upon composite materials to meet their ever-increasing demands for improved efficiency and lower costs. Composite materials also are used in automotive, recreational, military and defense applications, where the performance requirements may be even more demanding.
A significant drawback to the use of composite structures in aerospace applications, whether commercial or military, is the complicated and expensive tooling that is required for their fabrication, particularly when a seamless, hollow structure is desired. To form a seamless, hollow composite structure, the use of a mandrel or mold core is often preferred. The composite materials, generally fiber and resin, are laid up on the mandrel and cured by applying heat, time and pressure according to well-known methods. For many applications, the mandrel is a single use mold/tool that is destructively removed from the finished part either by a chemical process or by mechanical agitation.
Mandrels for composite structures are often made of plaster. Plaster easily pours into a mold and forms a solid structure but requires a significant curing time. Moreover, plaster is generally removed by mechanical agitation which can result in damage to the composite structure, after which the material is discarded as waste.
Other conventional materials used for making tooling such as mandrels include eutectic salts. These materials pose certain processing problems associated with removal of the materials from the cured parts, as well as with the disposal of the materials. Salt mandrels are brittle and must be cast into the desired shape while molten to avoid the need to machine them. Moreover, despite being soluble in water, eutectic salts produce corrosive, environmentally unfriendly waste streams when washed from the cured composite part.
An alternative method for producing a seamless, hollow composite part is to use an inflatable bladder as a mandrel within a reusable female mold form. Such a method is disclosed, for example, in U.S. Pat. No. 5,366,684. Upon inflation, the bladder presses the laid-up composite into the female mold form. The bladder process, however, is not useful for complex shapes and does not produce a composite structure with the same accuracy as a more conventional molded mandrel, particularly where the internal dimensions of the part are critical.
More recent improvements in mandrel materials provide organic and inorganic binders that are environmentally benign and water-soluble. The mandrel material is a composite blend including a matrix, such as sand, a binder, and water. One such binder is polyvinylpyrrolidone, or “PVP”. The composite blend is prepared to a desired consistency, formed into a desired shape and cured. The resulting mandrel is strong and lightweight and easily can be shaped and subsequently removed from cured composite parts. Additives may be added to enhance the functional characteristics of the finished tooling material. These types of mandrel materials are disclosed, for example, in U.S. Pat. Nos. 6,325,958 and 6,828,373, both of which are incorporated by reference herein.
Utilizing the above processes (excluding the bladder technique) mandrels are currently formed using similar techniques, which involve the use of either a pourable material, such as eutectic salt or plaster, or a compressible material such as PVA and aggregate or sodium silicate and aggregate. Each of these processes can be very labor and time intensive.
Eutectic salts mandrels require the salt to be melted at temperatures in excess of 350° F. and, once molten, must be manipulated into a mold where it must cool and set for extended periods of time, often causing burns unless special protective clothing is worn. Further depending on the complexity of the mold and exposed cross section, significant amounts of water and time are needed to remove the eutectic salt from the mold, which also creates a highly corrosive waste stream.
Plaster is a more user friendly material, in that it can be prepared at room temperature and poured into a complex mold, but it also has several disadvantages. Since plaster is formed from the hydration of dehydrated salts, it is prepared from a dry powder material that is combined with water, which must be agitated to ensure proper mixing of the powder and water. This agitation leads to the formation of air bubbles within the material that can form significant defects in the resultant mandrel. Further, the plaster must be allowed to set, which is controlled by the temperature that the plaster is mixed at, as well as by additives to the mixture. Since this is normally a manual process, set times are around 10 min and can take as long as 45 min. Once set, the mandrel can be removed from the mold, but is not ready for composite layup, since it still contains significant amounts of water. This water must be removed to a sufficient level so as not to react with the composite system when the part is brought to temperature. Plaster is extremely time and energy intensive to dry since it forms a dense egg shell like skin, which acts as a heat and water barrier. Once a finished composite part is formed, the plaster material is either partially or completely water insoluble. Mechanical methods are then used to remove the mandrel often damaging the composite due to delamination.
Prior works have attempted to produce a water soluble plaster material that aids in the removal of the core from the finished composite part, but has come at the cost of reduced strength in the core. Further since these water soluble plasters still incorporate large amounts of water to pour the material there is a significant amount of time and energy needed to cure and dry the core.
Existing compressed material mandrels are comprised of moist sand-like materials that are packed or compressed into a mold to form the required shape. These materials can be labor intensive or require expensive tooling since the materials must normally be packed at high pressure to ensure uniformity in the mandrel. Further, complex shapes are very difficult to form since the materials are not very flowable and, as such, don't tend to fill molds with reverse cavities. However, since these materials start out with a very low density and open porosity, they can be readily dried or cured using a variety of techniques, for example CO2 curing, hot gas infiltration, vacuum drying, microwave or oven drying.
While these prior art practices provide improvements that have shortened processing time and overall costs, the manufacturing cost of a composite structure is still relatively high. Consequently, there remains a need for a simplified method for manufacturing composite parts. In particular, a need exists for simplified methods of formation of mandrels and for removal of mandrels from a seamless, hollow composite part.